Recently, it has been increasingly demanded to restrain the consumption of fossil fuels in order to control global warming and protect the environment, which has affected various manufacturing industries. For example, automobiles, which are an indispensable part of transportation means in daily life and activities, are not exception. There is a demand to improve fuel economy by, for example, reducing vehicle body weight. It is not allowed, however, to simply reduce the vehicle body weight by neglecting product qualities. It is necessary to secure appropriate safety.
Many of the structural parts of an automobile are made of steel, in particular a steel sheet. For reducing the vehicle body weight, it is important to reduce the weight of the steel sheet. Instead of simply reducing the weight of the steel sheet, which is not allowed as mentioned above, the weight reduction must be accompanied with maintaining the mechanical strength of the steel sheet. Such demand becomes higher not only in the car manufacturing industry but also in various other manufacturing industries. Research and development efforts have been directed to a steel sheet that can have the same or a larger mechanical strength as compared to conventional one even when the sheet is made thinner.
In general, a material having a high mechanical strength tends to become lower in formability and shape fixability in shape formation work such as bending. It is difficult to carry out the process for forming such material into a complicated shape. One of the solutions to the formability problem is what is called “a hot pressing method (also referred to as hot stamping, hot pressing, die quenching, press hardening)”. In the hot pressing method, a material to be formed is heated temporarily to a high temperature (in an austenite region) and the steel sheet soften by the heating is formed by pressing. The steel sheet is then cooled. By using the hot pressing method, the material is once soften by heating to a high temperature so that the material is easy to be pressed. The mechanical strength of the material becomes larger due to a quenching effect during cooling after the shaping is completed. Accordingly, the hot pressing can provide a product having both a good shape fixability and a high mechanical strength.
When the hot pressing method is applied to a steel sheet, however, iron and other substances on the surface are oxidized to generate scales (oxides) due to heating to a high temperature of, for example, 800° C. or more. Accordingly, a descaling process is necessary after hot pressing to remove the scales, which deteriorates productivity. For the members and the like that require corrosion resistance, it is necessary to carry out anti-corrosion treatment and metal cover installation on the surfaces of the members after the shaping process. A surface cleaning process and a surface treatment process are also necessary, which further deteriorates productivity.
As an example of restraining such deterioration in productivity, a covering layer can be installed on a steel sheet. In general, various materials including organic and inorganic materials are used for the covering layer on a steel sheet. Among them, galvanized steel sheets that have a sacrificial protection effect on steel sheets are widely used for steel sheets for automobiles and other products because the galvanized steel sheets provide a good anti-corrosion effect and suitability to steel sheet production technology. However, this may cause to considerable deterioration in the surface properties because heating temperatures used in the hot pressing (700 to 1000° C.) are higher than the temperatures at which the organic materials decompose or the zinc boils so that the plating layer evaporates at a time of heating by hot press.
For this reason, it is desirable to use, for example, what is called an Al-plated steel sheet for the hot pressing that heats the steel sheet to high temperatures. The Al-plated steel sheet is a steel sheet having an Al-based metal cover that has the boiling point higher than that of an organic material cover or Zn-based metal cover. The Al-based metal cover can prevent scales from depositing on the surface of the steel sheet, which leads to omitting a process such as the descaling process and improving productivity. The Al-based metal cover also has an anti-corrosion effect so that the corrosion resistance of the steel sheet after coated with paint is improved. Patent Literature 1 listed below discloses a method for using an Al-plated steel sheet in hot pressing, the Al-plated steel sheet being obtained by covering a steel sheet having predetermined steel components with Al-based metal, as explained above.
In the case that the Al-based metal cover is applied, the Al cover is melted and transformed into an Al—Fe compound due to the dispersion of Fe from the steel sheet, depending on preheating conditions before hot pressing. The Al—Fe compound grows until the Al—Fe compound reaches to the surface of the steel sheet. The compound layer is hereinafter referred to as the alloy layer. The alloy layer is so hard that scratches are formed by contacting with dies during the pressing work.
That is because the Al—Fe alloy layer is intrinsically not smooth on the surface and is inferior in lubricity, comparatively. In addition, since the Al—Fe alloy layer is comparatively hard, the Al—Fe alloy layer tends to break, develop cracks in a plating layer, and come off in a powder form. Moreover, flaked materials from the Al—Fe alloy layer and coming-off materials by strong abrasion on the Al—Fe surface attach on the dies. The Al—Fe compound then adheres to and deposits on the dies, which leads to deterioration in the quality of pressed products. To prevent this, it is necessary to remove Al—Fe alloy powder adhered to the dies during maintenance, which is one of the causes for lowering productivity and increasing the cost.
Furthermore, the Al—Fe alloy layer is less reactive in phosphate treatment so that a chemical conversion coating (a phosphate coating), which is a treatment before electrodeposition painting, is not generated. Although the chemical conversion coating is nor formed, the Al—Fe alloy layer itself has a good coating adhesion ability with paint so that corrosion resistance after coated with paint becomes better if Al plating deposition amount is large enough. An increase in the Al plating deposition amount, however, tends to worsen the aforementioned adhesion to the dies. The adhesion occurs in the cases that the flaked materials from the Al—Fe alloy layer attach on the dies or the coming-off materials by strong abrasion on the Al—Fe surface attach on the dies, as described above. An increase in the lubricity of the surface coating makes an improvement for the case that coming-off materials by strong abrasion on the Al—Fe surface attach on the dies. On the other hand, this improvement effect is relatively small for the case that flaked materials from the Al—Fe alloy layer attach on the dies. To alleviate the adhesion due to the flaked materials from the Al—Fe alloy layer, it is most effective to lower the Al plating deposition amount. However, lowering the deposition amount causes deterioration in the corrosion resistance as described above.
To address this issue, Patent Literature 2 listed below discloses a steel sheet with an objective to prevent scratches from occurring during work. The Patent Literature 2 proposes that an Al-based metal cover is formed on a steel sheet having predetermined steel components, and, on the Al-based metal cover, there is formed a coating made of an inorganic compound containing at least one of Si, Zr, Ti, and P, an organic compound, or a complex compound thereof. For the steel sheet with such surface coating formed thereon, the surface coating still remains during pressing work after heating so that the surface coating can prevent scratches from forming during pressing. In addition, the literature claims that the surface coating can also act as a lubricant during the pressing work, which allows to improve formability. In reality, however, a sufficient lubricity cannot be obtained and a new lubricant or an alternative means is still desired.
Patent Literature 3 listed below discloses a method related to the hot pressing of a galvanized steel sheet. The method addresses the surface degradation due to evaporation of a galvanized layer on the surface. The method according to the Patent Literature 3 relates to the forming of a barrier layer of zinc oxide (ZnO), which has the high melting point, on the surface of the galvanized layer so that a lower portion of the galvanized layer is prevented from evaporating and draining off. The method disclosed in the Patent Literature 3, however, presupposes the galvanized layer and does not practically assume the Al layer because it teaches that lower Al concentrations are better although Al is allowed to be contained up to 0.4%. In addition, the technical problem to be solved in the literature relates to Zn evaporation. This phenomenon does not occur, of course, in the case of Al plating because Al has the high boiling point.
Patent Literature 4 listed below discloses a method in which a surface coating layer containing a wurtzite-type compound is installed on the surface of an Al-plated steel sheet and then the steel sheet is subjected to hot pressing. According to the Patent Literature 4 listed below, the installation of such surface coating layer improves in lubricity in hot state and in chemical conversion treatability. This technique is effective for improving lubricity and also corrosion resistance after coated with paint. According to an example in the literature, however, the improvement in the lubricity in hot state with this technique requires application of a relatively large amount of the wurtzite-type compound, i.e., the amount being at 2 to 3 g per m2.
Patent Literature 5 listed below discloses a method for obtaining a steel sheet for hot pressing that can restrain scale generation when heated before hot pressing and prevent plating materials from adhering to dies during hot pressing work. In the Patent Literature 5 listed below, the suppression of the scale generation during heating and the prevention of the plating materials from adhering to the dies during the hot pressing are achieved by means of installing a plating layer of Al—Zn based alloy on the steel sheet surface and the Al—Zn based alloy contains, Al: 20 to 95 mass %, Ca: 0.01 to 10 mass %, and Si. However, the Al—Zn based alloy plating layer disclosed in the Patent Literature 5 listed below contains Zn, which leads to metal embrittlement cracking during hot pressing work and also deterioration in spot weldability because Zn oxides are generated during hot pressing work.
Patent Literature 6 listed below discloses a method for efficiently manufacturing a hot-dip Al-plated steel sheet having less plating defects. In order to manufacture a hot-dip Al-plated steel sheet with less plating defects, according to the Patent Literature 6 listed below, a steel sheet, which is heated with predetermined conditions, is immersed, for a predetermined period of time, in an Al plating bath containing one or more of the elements of Mg, Ca, and Li. The manufacturing method according to the Patent Literature 6 listed below, however, does not intend to be applied to manufacturing steel sheets for the use of hot pressing. Accordingly, the properties of a manufactured steel sheet that are required during hot pressing work still need improvement. The Patent Literature 6 listed below also discloses the case in which Zn is added in the plating bath. Zn addition in the plating bath, however, leads to metal embrittlement cracking during hot pressing work and also to deterioration in spot weldability, which is the same as described above.